1. Field of the Invention
The invention relates to pressure-sensitive adhesive tape constructions which are useful for masking printed circuit boards (PCBs) at the high temperatures associated with wave soldering and solder flux reflow board assembly operations. These adhesive tapes, which comprise conductive acrylic polymeric microparticulate adhesive formulations with a special primer, provide tapes extremely resistant to tribocharging, thereby protecting electronic components from static charge buildup. In addition, the adhesive masking tape, upon removal from a PCB, does not contaminate the surface of the board with adhesive residue.
2. Description of the Art
The processes of wave soldering and solder flux reflow are commonly used for permanently attaching electronic components to printed circuit boards. Solder flux reflow operates at higher temperatures and heating of the PCB lasts longer than required for wave soldering.
Various methods are used to mask or cover areas of the board during soldering operations such as wave soldering and solder flux reflow. Masking prevents undesirable contamination of circuit boards by solder used to make electrical connections. It is known, for example, to achieve such masking by use of self-adhesive tapes based on high-temperature-resistant polyimide film coated with a silicone-based adhesive. However, the removal of such tapes from the surface of electronic assemblies causes tribocharging accompanied by static charges which can damage sensitive electronic components and can also cause contamination of the printed circuits by silicone.
Electrically conductive tapes are also useful for the masking purpose. Electrically conductive tapes do not tribocharge as readily as those made from insulating materials such as silicones. The use of conductive tapes, in printed circuit board assembly operations, therefore, will reduce the failure rate of electronic components.
Several different types of conductive tape are known for use at ambient temperatures. U.S. Pat. Nos. 3,104,985, 3,832,598 and 4,749,612 describe adhesive tapes with a coating of carbon black in a binder which is taught to dissipate electrostatic charges. Various patents also disclose multiple layer tape structures wherein one of the layers, usually a buried layer, is electrically conductive.
For example, Japanese Patent Publication J63012681-A discloses a tape with an intermediate, antistatic polymer layer situated between a polyolefin support and a rubber adhesive layer.
European Patent Publication EP 0422919-A2 discloses a tape having a layer of conductive particles or conductive foil surrounded by binder, situated between a polymer film support and a silicone adhesive. The use of a high temperature film support, polyimide, combined with silicone binder and adhesive, is stated to yield a tape which will perform well as a wave solder masking tape at temperatures unsuitable for earlier antistatic tapes, i.e., this tape will survive in a wave solder bath for at least about 5 seconds at 250.degree. C.
Antistatic or conductive tapes which rely on the use of conductive particles require high concentrations of these particles to provide sufficient electrostatic charge neutralization in order to provide the effect of the conductive particles at the surface of an otherwise insulative adhesive. Without surface activity, static free masking of electronic assemblies cannot be achieved, as charge transfer to the underlying layer of conductive particles requires a conductive pathway through the adhesive. However, high particulate concentrations often lead to loss of adhesion and undesirable transfer of contaminating adhesive material to the bonded surface. Adhesive transfer must be balanced against tribocharging; the use of additional polymeric binder will reduce transfer but may electrically insulate adjacent conductive particles and thus cause increased tribocharging.
The need for balance between particle loading and polymeric binder could be avoided by use of an inherently conductive adhesive layer. However, there is no known disclosure of either a wave solder or reflow solder masking tape using an inherently conductive adhesive in direct contact with the printed circuit board. Whether conductive in nature or not, most non-silicone adhesives will not survive the wave soldering process, and especially not the reflow solder process. Non-silicone adhesives are thus not generally useful for such an application.
Particulate adhesives are also known in the art, and have been coated on a variety of substrates and used primarily in applications requiring a low level of adhesion, e.g., repositionability. Such spheres and their use in aerosol adhesive systems having repositionable properties are disclosed in U.S. Pat. No. 3,691,140 (Silver). These microparticles are prepared by aqueous suspension polymerization of alkyl acrylate monomers and ionic comonomer, e.g., sodium methacrylate, in the presence of an emulsifier. The use of a water-soluble, substantially oil-insoluble ionic comonomer is critical to preventing coagulation or agglomeration of the microparticles. However, particulate adhesives disclosed in the prior art have all been useful as repositionable adhesives for such applications as Post-It.TM. brand notes, and other removable items. Pressure-sensitive tapes made with this type of adhesive are likely to be considered unsuitable for use as antistatic tapes due to their lack of conductivity, and ease of removal. Further, acrylic adhesives have not been considered to be heat resistant in nature.
U.S. Pat. No. 5,378,405 discloses conductive microparticulate adhesives, and antistatic tapes made therefrom. The conductivity is due to the presence of an ionic conductive material present on the surface of the microparticle. The antistatic adhesive tapes disclosed in this patent, while exhibiting extremely low tribocharging upon use and removal, also exhibit adhesive transfer when exposed to the high temperatures of molten solder.
The current inventors have now discovered an inherently conductive adhesive tape useful at the high temperatures required by wave solder and reflow solder processes . The synergistic behavior of a specific primer and an acrylic microparticulate adhesive provides a tape construction which is useful for wave soldering and reflow solder applications without the adhesive transfer or tribocharging problems of previously disclosed wave solder masking tapes.
Adhesive tapes of the invention comprise conductive acrylic microparticulate adhesives having polymer electrolytes on the surface of each adhesive particle which provides conductive particles which are useful as antistatic adhesive compositions.
Surprisingly such adhesives, when coated atop a primer containing both a phenolic constituent and a rubbery constituent exhibit high temperature resistance with low adhesive transfer when run through a wave solder bath at 260.degree. C. for several seconds. A further, unexpected improvement in high temperature performance occurs with the addition of selected antioxidant additives that appear to provide thermal stabilization to the adhesive formulation. Typical acrylate adhesives, having high temperature stability, include acrylate polymers, polymer electrolytes, thermal stabilizers and additives to enhance dissipation of electrostatic charge. Such compositions provide adhesives that perform satisfactorily at the higher temperatures demanded by solder flux reflow processing. Tape constructions comprise high temperature films, such as polyimide films, coated with a layer of the antistatic adhesive.
Adhesive tapes of the invention provide antistatic tapes which are extremely effective in dissipating electrostatic charge and may be used in sensitive applications without worry about adhesive transfer. These tapes provide excellent electrical properties essentially with substantially no adhesive transfer due to the excellent adhesion between the adhesive tape and the primer therefor, even at high temperatures.